1. Field of the Invention
This invention relates to the production of anhydrous alkaline metal hydrosulfites or dithionites from formates and sulfur dioxide. It specifically relates to production of sodium dithionite in an aqueous methanolic solution in which both sodium formate and sulfur dioxide are dissolved.
2. Review of the Prior Art
Hydrosulfites, also termed dithionites, are in demand as bleaching agents, such as for bleaching groundwood pulps. Zinc dithionite is being replaced by sodium dithionite because of the shortage and increasing cost of zinc dust to produce zinc dithionite and because of ecological objections to disposal of zinc-containing wastes. Sodium dithionite can be produced by electrolytic and borohydride procedures, but the most economical procedures for making a high-quality solid product increasingly use the formate radical as a means for reducing the valence of the sulfur atom.
This development began with U.S. Pat. No. 2,010,615 which discloses a method for producing anhydrous alkali metal dithionites by introducing gaseous sulfur dioxide into an aqueous methanol solution, which contains sodium formate and sodium carbonate and is held at a temperature below 30.degree. C., and then bringing the SO.sub.2 -methanol solution to the temperature at which sodium dithionite formation begins. In one Example, Na.sub.2 CO.sub.3 is 19.0% of sodium formate. This process requires a considerable excess of sodium formate to buffer the acidity of the solution and produces crystals of excessive fineness and low stability.
More than 30 years later, a succession of improvements based on sodium hydroxide as the source of alkali, were disclosed, particularly including U.S. Pat. Nos. 3,411,875; 3,576,598; 3,714,340; 3,718,732, 3,872,221; 3,887,695; 3,897,544; 3,917,807; and 3,927,190; Japanese Pat. Nos. 7003/68 and 2,405/71; and Belgian Pat. No. 698,247.
These improvements comprise adding sulfur dioxidecontaining methanol and an alkaline agent to an aqueous solution of an alkali metal formate and holding the resulting aqueous methanol solution at a reaction temperature above the dehydration point of the hydrated alkali metal dithionite in order to prevent the formation of crystals having water of crystallization occluded therewithin. The rate of addition must generally correspond to the rate of production of dithionite; if too rapid, the dithionite ion decomposes, thus reducing yield. The rate of addition therefore effectively controls productivity, measurable as weight of pure dithionite per unit of reactor volume per hour.
Japanese Pat. No. 7003/68 teaches absorbing sulfur dioxide in methanol to a suitable concentration and then gradually adding, in Example 1, separate aqueous solutions of sodium carbonate (25%) and sodium formate (42%) and, in Example 2, an aqueous alkaline solution (33% total concentration) combining sodium formate and sodium carbonate which is at 33% by weight of the sodium formate. The calculated yield is about 56% based on sulfur dioxide and 53% based on sodium.
U.S. Pat. No. 3,897,544 teaches in its Example 3 the suspension of sodium formate (11.0%) and of sodium carbonate (4.0%) in 80% aqueous methanol solution at 71.degree. C., adding an SO.sub.2 -methanol solution during 2 hours, and refluxing for an additional 5 hours to obtain anhydrous sodium dithionite, having a purity of 87%, at a yield to sulfur dioxide of 75%.
From the examples showing experimental results in the prior art, it appears that: (a) sodium hydroxide has been routinely selected as the alkali source; (b) sodium carbonate is seldom used but is often mentioned; and (c) in the few available examples in which sodium carbonate is employed as the alkali source, the yield is decidely inferior to those examples in which sodium hydroxide is used as the alkali source for producing sodium dithionite.
U.S. Pat. Nos. 3,576,598 and 3,887,695 teach absorbing sulfur dioxide in a water-miscible alcohol as a first feed solution, completely dissolving sodium hydroxide and sodium formate outside of the reactor in very hot water (160.degree. C.) as a second feed solution, and feeding these two solutions into a reactor which is held at 60.degree.-90.degree. C. and contains a small amount of the alcohol under superatmospheric pressure. High reactor concentrations are used to obtain high production per unit of reactor volume. U.S. Pat. No. 3,887,695 differs from U.S. Pat. No. 3,576,598 in that the former teaches recycling to the next batch of by-product methyl formate with the methanol as part of the reactant feed solution. The latter patent disclosures a productivity of 0.78 pounds of sodium dithionite production per gallon of reactor volume per hour, and the former patent discloses a productivity of 0.65.
U.S. Pat. No. 3,887,695 discloses a commercially valuable process that is highly advantageous with respect to productivity and simplicity. However, in order to obtain optimum productivity, it is necessary to dissolve all solids, by heating a mixture of water, sodium formate, and sodium hydroxide to a temperature (160.degree. C.) that is critical with respect to saturation and freezing-out of the alkali, and then to transfer the very hot, saturated solution from a dissolving vessel to the reactor. For this reason, the process is subject to difficulties in large-scale industrial production because of freeze-ups within supply pipes to the reactors from slight temperature drops and because of potential hazards from corrosion of equipment by the hot alkali and spraying of hot alkaline solutions upon operating personnel.
One way of minimizing the above difficulties is to compromise on the concentration of the alkaline solution at the cost of lowering the productivity from 0.65 to about 0.475 pounds of pure Na.sub.2 S.sub.2 O.sub.4 per gallon of reactor volume per hour. An improved process for obtaining the optimum productivity of U.S. Pat. No. 3,887,695 while obviating the hazards and the freeze-up difficulties that are inherent in forming and transferring a sodium hydroxide solution at 160.degree. C. is consequently highly desirable from a commercial viewpoint.